The foundry industry has long been accustomed to the processes associated with the removal of excess cast material from cast products. In the typical foundry industry, the pouring of molten cast into molds inevitably leaves an excess portion of cast material extending from the cast product subsequent to the cooling of the molten material. This excess portion, often termed a neck or riser, is formed as a result of molten cast remaining in the pour hole of the mold during the pouring and cooling process. Once the exterior mold is removed from the cast product, the cast material previously remaining in the mold pour hole becomes riser extending from the cast product. This riser must be removed from the casting in order to yield a finished cast product.
The foundry industry utilizes various forms of single stroke pneumatic impactors to fracture a riser from cast products. These pneumatic single stroke impactors typically comprise a longitudinal bore having a piston slidably mounted therein, which is mechanically connected to a slidably extendable impacting rod. The piston is urged to travel longitudinally within the bore via selective pressurization of the head or blind end of the bore by a high-pressure air supply in fluid connection with the bore through selectively actuated valves, thus selectively extending and retracting the impacting rod. However, in order to impart sufficient velocity and inertia to the impacting rod to fracture the riser from the cast material, the bore must be rapidly pressurized by the opening of the aforementioned valves. In operation, the operator of the impactor simply aligns the retracted impacting rod with the riser to be fractured and activates the appropriate valve to accelerate and extend the impacting rod so that the riser to be fractured is contacted by the rapidly extending rod. The impacting rod transfers a substantial amount of kinetic energy to the stationary riser, thereby fracturing the riser from the casting.
However, a common cause of impactor critical failure is the frequent occurrence of partial or complete misses of the target riser to be fractured by the impacting rod. In these circumstances, the piston and impacting rod maintain a substantial amount of kinetic energy, which the impactor must then absorb. Generally speaking, upon a miss of the target riser by the impacting rod, the piston continues to travel toward the head end of the bore at or near maximum velocity. Upon reaching the head end of the bore, the piston contacts the head end and comes to a sudden stop. This sudden stop increases material stresses and often results in critical fractures in either the piston or the bore assembly. Additionally, a similar problem occurs when an impactor is used to fracture relatively small risers and such from castings, as the piston and rod assembly is only minimally decelerated by the impact with the small riser. Therefore, the piston and rod continue to longitudinally travel through the bore subsequent to impacting a small riser, thus again contacting the head end and potentially causing damage to the impactor components.
In order to lessen these material stresses, manufacturers of foundry impactors have attempted to decrease the head end velocity of the piston and impacting rod via rapid repressurization of the head end of the bore proximate the end of the impactor stroke. In practice, this concept involves precisely timing the opening of a valve connecting a high-pressure air supply to the head end of the bore proximate the end of the stroke. In theory this practice effectively reduces the head end velocity of the piston and impacting rod; however, in practice this configuration produces numerous disadvantages and is nearly ineffective. Inasmuch as this configuration utilizes a substantial portion of the bore for the deceleration process, the output power of the impactor is substantially diminished, as the length of the power stroke must be reduced to accommodate the deceleration portion of the bore. Thus, in order to generate adequate impacting power using this configuration, a substantially longer bore is required, which directly translates into larger impactor dimensions. Additionally, the timing of the opening of the repressurization valve is critical to the safe operation of the apparatus, as improper timing can again result in critical failure of the impactor and possible injury to the operator. Further, inasmuch as this configuration rapidly pressurizes the head end of the bore to decrease piston velocity, the resulting piston velocity at the blind end becomes an issue, as the piston is urged to reverse it's longitudinal direction of motion and rapidly travel towards the blind end of the bore upon pressurization of the head end. Thus, deceleration of the piston at the blind end becomes an issue, if additional critical failures are to be avoided.